John, Most high precision surveying is now done by *laser theodolites* which are capable of measuring distances within a few wavelengths of light, with uncertainties of only plus or minus several millimeters, well within 2 ppm in many applications.
Gene. ---- Original message ---- >Date: Sun, 30 Jan 2011 12:14:10 -0800 (PST) >From: "John M. Steele" <[email protected]> >Subject: [USMA:49694] Re: [SI] A mole hill into Himalaya >To: "U.S. Metric Association" <[email protected]> > > For the US, it is up to each state. Many still use > the Survey foot, some have gone to the International > foot since the release of new geodetic data to the > NAD83 datum. > > Anyway, I defy you to survey the difference which is > 2 ppm. Long steel tapes, when corrected for > temperature and at proper tension are certified to > 0.02% (200 ppm). A little Googling turned up an > Australian standard for EDM equipment, plus/minus 5 > mm + 30 mm/km over length of baseline, so 35 ppm > over a kilometer. Surveying simply doesn't have the > precision to resolve the difference. >..
